1. Field of the Invention
The present invention relates to an alternator for a vehicle such as a passenger car or a truck.
2. Description of the Related Art
Because engine noises have been reduced in order to comply with the social demand of reducing noises outside and inside cars, fan noise of the alternator becomes comparatively loud and harsh.
On the other hand, the engine compartment has become narrower due to a slanted-nose-vehicle for improving driver's visibility and vehicle running resistance, and a tendency to increase in the passenger compartment of the vehicle. This causes a temperature rise of the engine compartment and the alternator, thereby increasing power loss of the alternator. Moreover, the alternator is required to increase the output power for safety devices and the like with a limited size and being mounted in a limited space. The size of cooling fan can not be increased because of increase in noise.
Therefore, it is necessary to provide a noise-reduced, small and powerful alternator for a vehicle to be used under high-temperature circumferences, at low production cost.
In general, to obtain an increase in the magnetomotive force, improvement in the magnetic circuit and a pair of inner cooling fans fixed to opposite ends of pole cores are the basic way of providing a small and powerful alternator. In such an alternator, fan noise is caused by interference of radially moving cooling air with the coil ends of the stator and by the fan in cutting air.
JP-U-3-7648 discloses a cooling fan that can reduce fan noise. In such a cooling fan, a plurality of fan blades are disposed circumferentially at different intervals to disperse frequencies of noises caused by interference of cooling air with coil ends of the stator and to reduce noises by providing the fan blades with curved surface that inclines to the counter-rotating-direction. Because one or two of the fan blades of the cooling fan is or are disposed at each of the claw poles of the pole cores, a comparatively a large number of small blades generate turbulent air flow, and the intended effect thereof can not be expected sufficiently. To the contrary, the cooling air can not flow smoothly, thereby decreasing the quantity thereof to allow temperature rise of the alternator. Thus, it becomes difficult to make the alternator compact and powerful.